1. Field of the Invention
The present invention relates to an image recording device and an image recording method for use in performing a recording process by discharging ink to a record medium after discharging ink from a plurality of line heads on the basis of encoder output depending on the conveyance a record medium and correctly superposing each image.
2. Description of the Related Art
Conventionally, for example, a full line image recording device in an inkjet system is well known as an image recording device for recording image (including characters) on a record medium such as a paper sheet etc. The full line image recording device is provided with line heads formed to have a length equal to the width of a record medium in a direction (main scanning direction) orthogonal to the direction of conveyance (sub scanning direction) in which the record medium is conveyed.
The line head is provided with a long nozzle array having a plurality of nozzles for discharging ink drops and arranged for each color of discharged ink. The line head for each ink color is arranged at a predetermined interval in the sub scanning direction with the ink discharge surface of the nozzles facing the record medium.
With the image recording device, a recording process can be performed on the entire surface of the record medium only by relatively moving the record medium and the line head having the nozzle array in the direction substantially orthogonal to the array direction of the nozzles.
Therefore, with the above-mentioned image recording device, the recording process can be quickly performed by a simple operation of not moving a carriage for holding a short recording head and not performing intermittent conveyance etc. of a record medium as with a serial image recording device.
However, on the other hand, the line head is more costly, has a lower yield, has lower reliability, etc. than the short recording head. An image recording device for solving these problems is provided with a line head having a long discharge nozzle array as a whole by arranging a plurality of short nozzle arrays in staggered form in a direction of the extension of the nozzle array.
The above-mentioned line head has the merits of a short nozzle array that excels in cost, yield, and reliability, and also has the merits of a line head. A well known image recording device as an example of the image recording device records an image according to an encoder signal output depending on a relative movement in the sub scanning direction to improve the record precision.
Furthermore, there is an image recording device proposed as having the function of not stopping an image recording process although the pulse cycle of an encoder output signal fluctuates by the speed variations of relative scanning, dirty scale or sensor configuring an encoder, etc.
For example, the Japanese Laid-open Patent Publication No. 2006-327099 describes the technology of transferring print data to a recording head according to an encoder signal output depending on the movement of a carriage provided with a short recording head in the main scanning direction.
The document discloses an image recording device provided with a device for holding information about the printing position of print data not transferred yet to perform a re-printing operation when the transfer of print data to a recording head is not completed in a predetermined transfer period.
In the Japanese Laid-open Patent Publication No. 2006-327099, the image recording device performs a re-printing operation when the print data is not completely transferred to the recording head within a predetermined transfer period. Since the device is provided with a unit for holding the information about the printing position of the print data for which transfer has not been completed, the re-printing operation can be performed in the next scanning process if the data transfer has not been completed.
In addition, the Japanese Laid-open Patent Publication No. 2008-68603 describes an image recording device having an encoder provided with slits at predetermined intervals, an encoder sensor for detecting the slits, and a control device for controlling the operation of recording an image depending on the pulse output from the encoder sensor.
The document discloses the image recording device including a measuring device for measuring the cycle of the output pulse of the encoder sensor, and a correction device for correcting the operation timing of the control device when he measured pulse cycle exceeds a predetermined range.
In the Japanese Laid-open Patent Publication No. 2008-68603, the operation timing or the output timing of a discharge trigger signal is delayed when the cycle of an encoder pulse is shorter than a predetermined value. Therefore, although the cycle of an encoder pulse is fluctuated by a dirty encoder scale, it hardly affects the operation timing of discharge etc., thereby maintaining high quality printing.
The above-mentioned image recording device provided with a line head, a record medium can be conveyed at a high speed of up to 1000 mm/s for recording. In this case, since there occurs a large load fluctuation especially in conveying a cut paper sheet, speed variations easily occur during the conveyance of a record medium.
Especially when the conveying speed changes for a higher speed, the cycle of an encoder signal as a reference of head drive becomes short. In this case, data transfer may not be completed for a recording head within a predetermined time or a faulty ink discharge may occur on the nozzles.
If the recording operation is performed by replacing the input encoder signal cycle with a longer drive cycle to avoid the above-mentioned problems when the conveying speed of a record medium is changed for a higher speed, a recorded image can be recorded after shifted behind in the direction of conveyance of the record medium.
Especially on an image recording device provided with a plurality of line heads for a plurality of colors in the direction of conveyance, the recording operation is performed on the record medium after shifted behind in the direction of conveyance at different position for the respective colors. Therefore, there occurs a dot position shift for each color for superposed printing, thereby causing undesired color variations.
If the average conveying speed of a record medium is sufficiently delayed as compared with the speed fluctuation (speed variations), there occurs no fluctuation to a higher speed. As a result, the problems that the data transfer to the recording head cannot be completed and that faulty discharge of the head occurs can be avoided. However, if the range of the sufficient delay for the speed fluctuation (speed variations) is too large, the throughput of the recording process can be degraded.